Studies are underway to determine the role played by recruitment of CFU-M into cell cycle in response to platelet depletion and depletion of CFU-M by ionizing radiation using HU and ara-C. The use of centrifugal elutriation is being optimized to both enrich for megakaryocytes for the maturation studies outlined in the grant as well as progenitor-rich, megakaryocyte-free fractions. The latter are being used in FCM and cell sorting studies to further characterize CFU-M in terms of mitotic and endomitotic activity. This work will provide important new information on the cellular mechanisms operating in the progenitor compartment which account for a steady supply of polyploid megakaryocytes. As a model for studying the effect of suppression of platelet production on megakaryocyte proliferation we are using immune-induced thrombocytopenic mice at a time when their platelet counts have recovered above control levels. Since it is known that thrombopoietin production is shut off at this point, it serves as a reasonable time to examine the effect of decreased platelet production on CFU-M and their cycling state.